1. Field of the Invention
The present invention relates to a wireless communication system capable of wireless communication between a base station and a communication terminal by detecting a usable wireless link and by changing the wireless link used for connection and a method of wireless communication, and particularly to a technique for effectively utilizing resources of the wireless link.
2. Description of the Related Art
Various types of wireless access systems such as mobile phone, wireless LAN, PHS and the like have spread, and a wireless MAN service such as WiMAX (Worldwide Interoperability for Microwave Access) is going to start.
In order to effectively utilize such access systems, research and development have been made on a technique to handover different types of wireless networks in a seamless manner (See, G. Wu, P. Havinga and M. Mizuno, “MIRAI Architecture for eterogeneous Networks,” IEEE Comm Mag., pp. 126-134, 2002; and M. Inoue, K. Mahmud, H. Murakami, M. Hasegawa and H. Morikawa, “Novel Out-of-Band Signaling for Seamless Interworking between Heterogeneous Networks,” IEEE Wireless Commun., Vol. 11, No. 2, pp. 56-63, 2004). Also, as disclosed in H. Harada, “Software Defined Radio Prototype toward Cognitive Radio Communication Systems,” IEEE Dyspan 2005, Vol. 1, pp. 539-547, 2005, a software wireless technique capable of connection with different wireless interfaces in order to switch a different wireless system has been studied.
Based on these techniques, by switching to an optimal system according to a situation of a wireless network, more efficient use of limited radio resources (frequency, wireless infrastructure) and network resources will become possible. Also, as a result, total throughput and capacity are improved, leading to improvement efficient frequency usage.
In addition, for users, it becomes possible to communicate through an optimal wireless access all the time. For example, communication can be made through a mobile phone supporting high-speed movement during travel, while communication can be automatically switched to a wireless LAN through the inexpensive Internet when at home. A Cognitive Wireless Cloud advocated by the applicant of the present application is disclosed in Masahiro Kuroda, Yoshitoshi Murata, Hiroshi Harada, Shuzo Kato, “Cognitive Wireless Cloud (1)˜Architecture˜,” IEICE Search Report, Technical Committee on Software Radio, March 2007. The Cognitive Wireless Cloud is cognitive wireless architecture in which a cognitive wireless terminal capable of connecting to various types of networks through a plurality of wireless access means autonomously collects information of the networks available at the site, obtains QoS information of each wireless accesses and networks on a real-time basis and optimizes capacity and an error rate of the entire network.
Considering an environment in which a large number of radio waves are present, the total capacity is improved if a complex system of the network as a whole including the terminal is in the optimal state. As a result, a bit rate per frequency is improved, and the frequency use efficiency is improved. The cognitive wireless network can be considered as a technique to form such an optimal state while switching various types of wireless systems.
The problem relates to how integration of the plurality of radio resources in such a cognitive wireless network is achieved. For example, a link aggregation technique that integrates communication bands of a plurality of wireless links in order to improve a throughput of communication has been known. However, with the prior-art link aggregation technique, dynamic selection in a plurality of frequencies according to the use situation over time is not possible, and an effect in the cognitive wireless network is limited.
As a technique relating to the present invention, a technique relating to collection of wireless information has been examined by the applicants of this application. For example, a technique for a terminal to autonomously recognize a usable network is disclosed by Goh Miyamoto, Kentaro Ishizu, Mikio Hasegawa, Yoshitoshi Murata, “Cognitive Wireless Cloud (2)˜Data collecting method for discovering radio resources˜,” IEICE Search Report, Technical Committee on Software Radio, March 2007. A technique to obtain the respective network QoS information on a real-time basis is disclosed in Yoshia Saito, Mikio Hasegawa, Yoshitoshi Murata, “Cognitive Wireless Cloud (3)˜High-speed end-to-end QoS measuring method˜,” IEICE Search Report, Technical Committee on Software Radio, March 2007. In addition, a technique to make efficient radio resource reservations for seamless handover that can deal with high-speed movement is disclosed in H. N. Tran, M. Hasegawa, Y. Murata, “Resource Reservation Scheme for Mobile Users in Cognitive Wireless Cloud,” IEICE Search Report, Technical Committee on Software Radio, March 2007.
As a prior-art patent document relating to the present invention, an integrated wireless communication system is disclosed in Japanese Laid-Open No. 2008-211583. In this patent application, a plurality of wireless systems constituted by base stations accommodating terminal stations are connected to a network, respectively, and an information management server is installed therein. The server receives indication as to what type of wireless signals such as wireless communication standards, band widths, frequency channels, transmission power and the like are transmitted by each base station and at what location from each base station and creates a database. Then, in the information management server, a frequency to be used by each base station is determined and notified. A characteristic of this technique is that cognitive wireless that can optimize a frequency to be used is realized without direct wireless signal detection.
However, with the technique of Japanese Laid-Open No. 2008-211583, only the frequency that can be used from location information of the base stations is determined, and favorable communication cannot be necessarily realized by a communication terminal in an actuality. Also, communication quality required by the communication terminal is not necessarily ensured.
Japanese Laid-Open No. 2003-152732 discloses a software wireless communication device. This software wireless communication device converts a received radio wave by an analog-digital converter once and then performs demodulation or the like by a digital signal processor or a microprocessor in software. A plurality of frequencies or wireless methods can be dealt with by switching the software.
Japanese Laid-Open No. 2007-184850 discloses a cognitive communication system in which a database device maintaining information on a wireless channel use state is connected to each base station provided in the respective wireless access networks, and a database device having a database of frequencies used by the existing wireless access networks, integrating the database devices is installed on each of the base station sides. In order to manage wireless access devices with different communication methods or management methods in an integrated manner, common use of the functions and a communication interface between different devices are required. However, with the prior-art techniques, such functions are realized by a device developed specially for the wireless system, and the functions in the device are different and moreover, common use of the interface between the devices is not possible, which leads to a problem that general-purpose devices cannot be developed.